Spread spectrum communication systems use direct sequence spread spectrum (DSSS) codes for spectrum spreading channel signals communicated over respective communication channels and over a common channel bandwidth. Prior to transmission, an input signal having a narrow frequency spectrum, is spectrum spread by mixing the input signal with the DSSS code for providing a spread spectrum channel signal that has a spread spectrum extending in frequency over a wide channel bandwidth. The power spectra density of the communicated channel signal is spread over the channel bandwidth and is difficult to acquire without advance knowledge of the spreading code. As such, the communication system uses code division signaling for maintaining cochannel isolation of the multiple channel signals communicated over the channel bandwidth. Signal components of a channel signal are spread over the channel bandwidth providing low power components across the bandwidth rendering the isolation and reception of the input signal difficult without advance knowledge of the spreading code for correlation with the channel signal of interest. In order to reconstitute the spectrum spread signal upon reception, the channel signal is despread by correlating the received channel signal by a replica code generated in the receiver.
In a transmitter, spectrum spread channel signals are superimposed as an output signal that modulates a carrier for communicating by code division a plurality of channel signals, each of which is spectrum spread by the respective spreading code. Hence, each channel uses a respective spreading code, and hence, each receiver is equipped with a respective spreading code used for isolating a respective channel signal so that each receiver can acquire one and only one respective channel signal. During acquisition in the receiver, code phase is determined using the autocorrelation properties of the spreading code. When the receiver replica code is phase shifted in time to coherently match the code phase of the incoming channel signal, the signal is reconstituted and can be detected during code phase lock. When the code phase shifts, there is a drop in the level of the detection signals, and hence a lock drop preventing acquisition of the channel signal. Hence, the receivers automatically adjust the code phase of the replica code to dynamically match the code phase of the invention channel signal so as to maintain code lock during continuous reception of the channel signal. In the event the replica code has a large code phase differential, coherently code phase lock is lost, and the signal cannot be reconstituted nor detected.
The use of spreading codes have long been used to isolate channel signals so that only receivers with predetermined codes can only receive channel signals spectrum spread by the particular respective codes. In practice, each user or receiver is assigned apriori a respective unique spreading code. In some situations, a common channel signal can be broadcast to a plurality of receivers all having the same identical spreading code, such as in the broadcast of a GPS signal from a satellite to a plurality of GPS receivers each having the same spreading code for respective coherent reception. That is, the spreading code is used to broadcast a DSSS channel signal to a group of users all having a receiver generating a replica of the same spreading code for receiving the broadcasted signal. Once the grouped receivers are equipped with the same replica code, all user receivers will receive the same information at all times. In the presence of a jamming signal, all receivers will be equally jammed and communications will be equally jammed amongst all of the users. In clandestine or black out communications, it may be desirable to send a communication to a selected segment of the group and not to others of the group at various times. Such selective communications can be used in field operations where a first group is a friendly or desired group of recipients and the second group is a hostile or undesired group of recipients. Such selective communications during the broadcast of a spread spectrum signal could be used to communicate with the entire group at one time, and a preferred portion of that group at another time. Conventional broadcast spread spectrum communication systems using a single spreading code prevents time variable selective communications to a portion of a group of receivers. These and other disadvantages are solved or reduced using the invention.